1. Field of the Invention
The invention relates to the production of stable binder-free high-purity moldings composed of metal oxides, in particular pyrogenic metal oxides, and their use.
2. Description of the Related Art
Pyrogenic metal oxides are extremely finely divided and have high specific surface areas, defined, spherical primary particles having a defined surface chemistry and no internal surface area (pore). Furthermore, they have a very high chemical purity.
In view of the properties just outlined, pyrogenic silicon dioxides, for example, are attracting increasing interest as supports for catalysts (D. Koth, H. Ferch, Chem. Ing. Techn. 52, 628 (1980)).
However, owing to the particular finely divided nature of pyrogenic metal oxides, the production of moldings from these pyrogenic metal oxides is difficult. The production of moldings from metal-oxidic powders is generally carried out by pressing or extrusion using binders and lubricants in order to obtain stable moldings. The binders and lubricants are inorganic and organic additives.
Inorganic additives such as magnesium stearate remain in the moldings produced in the form of inorganic compounds such as magnesium oxide. Organic additives, too, can lead to contaminants such as carbon in the production process of the moldings. The desired very high purity of the pyrogenic metal oxides used, e.g. pyrogenic SiO2, is therefore lost in the moldings produced.
Another source of contamination is the production process itself. It is known that process steps such as milling produce contamination by abrasion on milling vessels, milling tools and milling balls and this contamination is introduced into the material being milled. This also occurs when milling vessels, milling tools and milling balls consist of hard materials such as Si3N4, ZrO2, ZrSiO4, Al2O3, and can be explained by the fact that the pyrogenic metal oxides are usually themselves materials having a high hardness of individual particles, e.g. aluminum oxide, silicon dioxide, titanium dioxide and zirconium dioxide.
EP 72390 describes the production of pressed bodies from a mixture of pyrogenic metal oxides, water, silica sol and a pressing auxiliary. A polyfunctional alcohol (e.g. glycerol) is used as auxiliary.
Mixing of pyrogenic silicon dioxide with kaolin and/or graphite, sugar, starch, urea, and aqueous waxes is known from EP 327722. Pressed bodies can be produced using punched presses, eccentric presses, extruders, rotary presses or compactors. According to EP 327815, an analogous procedure is employed but using pyrogenic silicon dioxide/aluminum oxide mixed oxide instead of pyrogenic silicon dioxide.
EP 393356 describes the production of pressed bodies from pyrogenic silicon dioxide, urea, methylcellulose and/or magnesium stearate, graphite, aluminum stearate and water.
EP 807615 discloses a process for producing pressed bodies comprising pyrogenic silicon dioxide, methylcellulose, microwax and polyethylene glycol and water. The pressed bodies usually have contents of 50-90% by weight of silicon dioxide, 0.1-20% by weight of methylcellulose and 0.1-15% by weight of microwax and 0.1-15% by weight of polyethylene glycol.
EP 916402 A1 describes the production of extrudates having a pore volume of from 0.5 to 1.8 ml/g from pyrogenic silicon dioxide. The starting mixture contains water and pyrogenic silicon dioxide with addition of methylhydroxyethylcellulose, wax and polyethylene glycol. The composition obtained by mixing is shaped in a screw extruder to form extrudates.
DE-A 10247314 discloses moldings based on silicon dioxide and/or titanium dioxide which additionally contain glass fibers. The moldings are produced by homogenizing pulverant silicon dioxide and/or titanium dioxide with glass fibers, methylhydroxypropylcellulose, wax emulsion or polyethylene glycol, polysaccharide and polyethylene oxide with addition of water. The resulting composition is shaped to form extrudates.
DE 3912504 discloses a process for producing pressed bodies based on pyrogenic silica, in which aluminum stearate, magnesium stearate and/or graphite are used as lubricant and urea and methyl cellulose are used as pore formers.
According to DE 4142898, it is possible to produce stable moldings from pyrogenic silica and aqueous-alcoholic ammonia solution. On the other hand, a pure aqueous ammonia solution is not successful. The high proportion of aqueous-alcoholic ammonia solution makes the mixture to be shaped strongly alkaline. The use of alcohol produces the risk of carbon contamination in the resulting catalyst support. According to DE 4142902, stable moldings can be produced from pyrogenic silica and ammonia solution or from pyrogenic silica and a silica sol containing alkali metal only when the shaped bodies are subjected to hydrothermal treatment. In the case of addition of ammonia, the mixture is once again made very alkaline. It is known that this excess of base (pH>10) leads to partial dissolution of SiO2.
The documents mentioned show that the production of stable moldings has hitherto not been possible without metallic or organic additives or additional strengthening steps. For this reason, only moldings which have a not inconsiderable proportion of contamination have been known in the field of support materials for catalytic applications. The patent texts US 2004106835 and WO 2006052688 may be mentioned as examples. According to US 2004106835, the total contamination by the elements Mg, Ca, Na, Al and Fe in an SiO2 catalyst support is at best 407 ppm. The SiO2 support which can be produced as described in the patent text WO 2006052688 contains less than 150 ppm of Mg, 900 ppm of Ca, 900 ppm of Na, 200 ppm of Al and 40 ppm of Fe.